Automotive and aerospace coatings must be of sufficient hardness and flexibility to be able to endure a wide variety of harsh environments including, for example, exposure to impact and corrosive materials. Further, a single coating composition has not been found that is capable of coating both metal and flexible polymeric substrates while maintaining long lasting adherence to both materials, particularly where, as is the case of a polymeric automotive bumper, the polymeric substrate may be flexed in use. Automotive and aerospace coatings are also expected to enhance and control the visible appearance of the substrate, especially with regard to color and gloss. At the same time, due at least in part to the introduction of the Clean Air Act of 1970, coatings formulators are also under pressure to reduce VOC, which translates to the solvent demand of the coating composition, to as low a level as technological advances will allow. One-component and two-component, isocyanate-based coating formulations are well-known and are widely used in the industry. Acrylic functional polyols, particulary the JONCRYL® or SCX™ acrylic polyols from S. C. Johnson, the Paraloid® acrylic polyols from Rohm and Haas, and the Reactol™ acrylic polyols from Eastman have long been the standard building block for two component automotive, aerospace, and industrial coatings where hardness and exterior durability are critical measures of utility (CMUs). Nonetheless, they still suffer from a high solvent demand in order to achieve a suitable application viscosity and there are limitations with formulating these resins into coatings, especially when low application viscosities are required. Reactive diluents are often used in combination with these resins to further lower the VOC of these coatings.
While reactive diluents may allow for the coating to be formulated at a desired VOC, they also have disadvantages in their use. Most reactive diluents are only di-functional and have low molecular weights, which often leads to a higher isocyanate demand and increased brittleness in the resulting coating. Addition of reactive diluents also tends to slow dry times and the diluent may fail to incorporate. Various acrylic polymers have been combined with polyester polymers as suggested in U.S. Pat. No. 4,766,177. U.S. Pat. Nos. 4,888,441; 4,902,821; 4,922,002; 5,023,367; (all of which patents are incorporated herein by reference) disclose linear, low molecular weight, polyester-based polyols which can been used with resins in the manufacture of coatings and have been reported to overcome some of the above-mentioned problems. Linear, low molecular weight, polyester-based polyols can provide relatively lower viscosity and maintain comparatively high solids content in comparison with most available acrylic resins. The linear polyols described in the prior art have an additional disadvantage in that they are only di-functional. This limits their ability to crosslink, which inevitably can compromise both the appearance and durability of the resulting coating under accelerated weathering (QUV) conditions. It would be highly desirable to provide new coating compositions, which are inherently low in viscosity and allow for minimal or no solvent addition (lower VOC) when applied to the surface of a substrate, but which are compatible with a variety of materials and exhibit the excellent attributes and properties associated with two component coatings, such as superior compatibility and applicability, a high (pendulum, pencil) hardness and better direct impact resistance, flexibility, appearance and gloss retention, and the like. The compositions of the present invention provide such properties and attributes.